In conventional technologies, progression and severity of a disease are understood by comparing and interpreting medical images of the same patient that are obtained at different periodically times so that treatment strategy can be determined.
For example, when a medical image photographed in a recent examination (recent image) is compared with another medical image photographed in a previous examination (past image) for interpretation, the recent image and the past image are displayed at a time by use of an image display software program having a function of loading multiple images in and displaying the images on a screen.
In addition, Patent Document 1 discloses an image diagnosis assisting apparatus that extracts a marker such as an abnormal shadow from a medical image and superimposes the extracted marker on the medical image, thereby facilitating understanding of the progression or severity of a disease from comparison and interpretation of a recent image and a past image.
In the comparison and interpretation, it is important to compare sectional images of substantially the same position of a patient, and registration should be performed on medical images of different photographing times. Furthermore, recent medical image diagnosis apparatus including X-ray CT apparatus is capable of creating three-dimensional medial images from sectional images obtained by photographing a patient, and thus registration is required for the three-dimensional medical images.
As methods commonly used for the registration between images, repetitive registration adopting image similarities and registration adopting landmarks are well known.
With the repetitive registering method adopting image similarities, the registration between the images is repeatedly performed by using image similarities such as similarity coefficients (see Non-Patent Document 1, for example). More specifically, dozens of local areas are determined in each of the three-dimensional medical images that are to be compared, and then correspondence between the local areas of the three-dimensional medical images is determined so that image similarities can be calculated for each pair of local areas. Furthermore, processes such as parallel shift, rotation, scaling (resizing), and shape altering are repeatedly performed on the images in such a manner to increase the image similarity. The registration of the three-dimensional medical images is thereby achieved. This repetition of the processes, however, requires a long time, and thus the registration between the three-dimensional medical images cannot be performed at high speed in accordance with the repetitive registering method incorporating image similarities.
On the other hand, with the landmark registering method, a pair of landmarks for the recent and past images are selected from extracted landmarks of the recent image and of the past image to perform the registration. In comparison with the repetitive registration adopting image similarities, the processing time can be shortened by adopting linear optimization for the registration of landmarks. For example, Non-Patent Document 2 discloses a technology of extracting branching points of the lung blood vessels or trachea as landmarks from chest CT images. By using the extracted branching points of the lung blood vessels or trachea as landmarks in accordance with this technology, the registration of the recent and past chest CT images can be established at high speed.
Patent Document 1:Japanese Patent Application Laid-open No. 2005-334219    Non-Patent Document 1:Jeongtae Kim, Jefferey A. Fessl, “Intensity-based image registration using robust correlation coefficients”, IEEE Transactions on Medical Imaging, Vol. 23, November 2004, pp. 1430-1444    Non-Patent Document 2:Hidenori Shikata, et al. “Algorithm for Localizing Points of Pulmonary Vessels for Non-rigid Registration in Lung”, IEICE Transactions, Vol. J85-D-II, No. 10, pp. 1613-1623, 2002